1. Field of the Invention
This invention relates to a two-cycle engine with at least one pair of cylinders of which power strokes begin alternately at each 180.degree. of crankshaft rotation.
2. Prior Art
In conventional two-cycle engines, blowby gas tending to flow from a combustion chamber through an exhaust port is forced back into the combustion chamber by a reflected pressure wave of the exhaust gas produced by the provision of an exhaust chamber formed in an exhaust passage, thereby improving charging efficiency of the fuel-air mixture in the combustion chamber to achieve an increased power output. As is known, the reflected wave is composed of alternate positive pressure and negative pressure portions. Therefore, to achieve the improved charging efficiency mentioned above, it is necessary that the positive pressure portion of the reflected wave should reach the exhaust port to efficiently force the blowby gas back to the combustion chamber during a time period between the closing of the scavenge port and the closing of the exhaust port. However, this can be done only in a predetermined range of the engine speed which range is determined by the length of the exhaust chamber. Therefore, in those ranges of the engine speed other than such a predetermined range, the charging efficiency can not be improved, which results in failure to obtain a desired power output. To make matters worse, when the negative pressure portion of the reflected wave reaches the exhaust port during the time period between the closing of the scavenge port and the closing of the exhaust port, the blowby gas is positively drawn into the exhaust passage through the exhaust port to thereby lower the charging effiencey.
In order to overcome this difficulty, it has been proposed in Japanese Utility Model Publication No. 59-41290 to provide a sub-chamber in the exhaust passage, and the communication of the sub-chamber with the exhaust passage is controlled by means of a valve in accordance with the engine speed. More specifically, when the positive pressure portion of the reflected wave is not to arrive at the exhaust port during the time period between the successive closings of the scavenge and exhaust ports, the valve is opened to cause the gas in the sub-chamber to flow therefrom into the exhaust passage to attenuate the reflected wave travelling toward the exhaust port, thereby preventing the charging effiency from being lowered.
Japanese Utility Model Application Laid-Open (Kokai) No. 51-163314 discloses another alternative to overcome the above difficulty by providing a valve in the exhaust passage which valve is operated to move between closed and open positions to vary the effective length of the exhaust passage in accordance with the engine speed, so that the positive pressure portion of the reflected wave can reach the exhaust port over a wide range of the engine speed before the exhaust port is closed.
The above conventional two-cycle engines have been found not entirely satisfactory, however, in the following respects. First, in the case where the two-cycle engine is of the multi-cylinder type, either sub-chambers or actuators for operating valves must be provided in respective exhaust passages. This increases the size and weight of the exhaust pipe. In addition, it is quite possible that the use of a plurality of valve actuators affects the reliability of the engine.